Both towers in Manhattan's financial district will remain closed for at least a week, officials said, with severe disruption to many businesses and the 50,000 workers in the second tallest buildings in the world.
In an effort to avoid a major blow to the region's economy and the world's financial markets, office space has been found within the seven-building complex for the several exchanges that set prices on commodities ranging from gold and cotton to orange juice. One estimate said closure of the centre would cost dollars 20m ( pounds 14m) a day in trades.
But the commodities and futures markets are far from being the only ones affected. The overall cost to all the other businesses in the centre will be enormous. One Japanese bank estimated recently that closing the centre would cost it dollars 20m a day in lost business.
Analysts said the explosion was a catastrophe for the insurance industry, which expects large claims from companies unable to conduct business.
The towers were pronounced structurally sound despite gaping holes in the basement floors and a 100-foot by 200-foot hole in the ground below, but officials said the buildings could not be reopened until the safety systems had been repaired.
By last night the police had had 50 telephone calls claiming responsibility for the blast, but no details had been made public. 'Any specifics would undermine the investigation,' said New York's police commissioner, Raymond Kelly.
However, at least one of the calls was from what Mr Kelly described as a 'Serbian liberation front' and several others apparently linked the bomb to the war in the Balkans. Talks on a peace plan for Bosnia are due to open at the United Nations here today.
James Fox, the FBI's New York director, said the huge size of the blast made it unlikely that a lone attacker was responsible, and he raised the possibility that it might have been the work of a drug ring. 'There are drug dealers who are very upset with the US government.' The finding of nitrate near the blast site indicated that TNT or dynamite were possibly involved.
Security was tightened in hundreds of skyscrapers throughout the city, as traffic in the financial district returned slowly to normal. Special precautions were being taken at the Statue of Liberty, the Empire State Building and railway stations and airports.
Since the blast many questions have been raised about fire codes and safety systems in the centre.
According to some reports, the Port Authority was warned at least six years ago by its terrorism task force that it needed to take steps to protect the complex from a car bomb, including banning public parking in the basement garage, However, it seems that almost none of the recommendations was implemented.
Other proposals included better protection for the buildings' back- up generators and installing battery-powered emergency lighting on the stairs which were used in the evacuation of the buildings on Friday.
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